1. Field of the Invention
The invention relates to a method of so-called "ultrasonic" peening using a mist of microbeads inside a chamber and more particularly to a method for peening large sized annular surfaces on thin parts.
2. Summary of the Prior Art
It is known practice for the surfaces of metal parts to be peened by blasting with microbeads. By impacting the surface of the metal part with a small angle of incidence with respect of the perpendicular to this surface and with sufficient kinetic energy, the microbeads cause permanent compression of the surface to a small depth. This compression inhibits the initiation and propagation of cracks at the surface of the metals part and thus improves the fatigue strength of that metal part.
The microbeads are commonly ball bearings. They are usually made of ceramic or steel with a diameter in the range 0.2 mm to 4 mm. Peening is performed inside a closed booth using nozzles supplied both with compressed gas and with microbeads, the compressed gas propelling the microbeads towards the metal part.
In the aeronautical industry, thin metal parts of large dimension are manufactured. A number of difficulties are encountered in peening these thin metal parts. For example, large metal parts require large closed booths for peening operations.
Peening is often light in order not to deform the thin metal parts. This is because thin metal parts are unable, without deforming, to absorb the loadings caused by the compressive stresses resulting from the microbeads of heavy peening.
There is an optimum exposure of the metal part to peening for imparting the best strength. However, such optimum exposure to peening is difficult to achieve because peening nozzles are difficult to adjust and are not stable. Insufficient peening does not give the anticipated strength, but additional peening to achieve the optimum exposive may lead to excessive peening which causes irreparable surface damage to the metal part, with a resultant reduction in the strength of that metal part.
French Patent 2 689 431 discloses a modified method of ultrasonic peening which involves sustaining a "mist" or microbeads inside a chamber. The mist of microbeads is sustained by a vibrator operating at frequencies in the order of 20 kHz. The chamber is open and the metal is pressed against the opening of the chamber. Peening is by microbead impact on the metal part. The chamber and the metal parts are given a relative movement aimed at causing the chamber to pass over the entire surface of the metal part to be peened. The patent also discloses how to peen circular parts such as shafts.
The term "mist" is used by analogy with the mists formed by minuscule water droplets. In ultrasonic peening, the microbeads have speeds which are random both in magnitude and in direction, which causes the microbeads to ricochet off each other, off the walls of the chamber and off the surface of the part in contact with the mist of microbeads.
FR2689431 provides examples of massive parts capable of absorbing, without deformation, the loadings resulting from peening in accordance with its method. However, the method of FR 2689431 does not allow the peening of thin circular metal parts, as these circular metal parts begin to deform very early in the peening operation. Even if the surface of the thin circular part is peened uniformly, these deformations are only partially absorbed at the end of the peening operation as they are plastic and non-linear deformations of the material. In addition, the method of FR 2689431 demands, if uniform peening is to be obtained, that peening be halted precisely when the circular metal part has made one revolution. Otherwise, over rotation would produce a localized excess of peening on a zone of overlap, whereas under rotation would give rise to a gap of localized lack of peening which would be difficult to correct without giving rise to excessive peening in a region immediately adjacent the original gap of lack of peening.
A first problem, therefore, is that of peening thin and circular parts whose dimensions exceed those of the peening chamber, without deforming the parts. A second problem is that of guaranteeing uniform peening across the entire surface that is to be peened.